Railway well cars may be considered as upwardly opening U-shaped channels of a chosen length, simply supported on a pair of railcar trucks. Although single unit well cars are still common, there has been a trend in recent years toward articulated, multi-unit railcars which permit a relatively larger load to be carried on fewer railcar trucks.
Contemporary well cars may carry a number of alternative loads made up of containers in International Standards Association (ISO) sizes or domestic sizes, and of highway trailers. The ISO containers are 8'-0" wide, 8'-6" high, and come in a 20'-0" length weighing up to 52,900 lbs., or a 40'-0" length weighing up to 67,200 lbs. Domestic containers are 8'-6" wide and 9'-6" high. Their standard lengths are 45', 48' and 53'. All domestic containers have a maximum weight of 67,200 lbs. Recently 28' long domestic containers have been introduced in North America. They are generally used for courier services which have lower lading densities. The 28' containers have a maximum weight of 35,000 lbs.
Two common sizes of highway trailers are, first, the 28' pup trailer weighing up to 40,000 lbs., and second, the 45' to 53' trailer weighing up to 60,000 for a two axle trailer and up to 90,000 lbs. for a three axle trailer. It is advantageous to provide well cars with hitches at both ends. This permits either a single 53' three axle trailer to be loaded in either direction, or two back-to-back 28' pup trailers to be loaded.
The wheels of a trailer can rest in the well, with the front, or nose of the trailer overhanging the car end structure at one end or the other of well car unit. A second trailer may rest in the well facing in the opposite direction. Alternatively, shipping containers, typically of 20 ft., 28 ft, or 40 ft lengths may be placed in the well, with other shipping containers stacked on top. Further, well cars may carry mixed loads of containers and trailers.
Whichever the case may be, a well car is required to withstand three kinds of loads. First, it must withstand longitudinal draft and buff loads inherent in pulling or pushing a train, particularly those loads that occur during slack run-ins and run-outs on downgrades and upgrades. Other variations of the longitudinal load are the 1,600,000 lbs. squeeze load and the 1,250,000 lbs. single ended impact load. Second, the well car must support a vertical load due to the trailers or shipping containers it carries. Third, it must be able to withstand lateral loading as the well car travels along curves and switch turn-offs. It is important to carry these structural loads while at the same time reducing the weight of the railcars themselves, first to permit a greater weight of freight to be carried within the overall maximum car and load weight limit, and second to reduce the amount of deadweight that must be pulled when the car is empty. Third, a lighter car may be less costly to build.
The U-shaped section of the car is generally made up of a pair of spaced apart left and right hand side beams, and structure between the side beams to support whatever load is placed in the well, and to carry shear between the sills under lateral loading conditions.
In earlier types of well car the side sills tended to be made in the form of a single, large, beam. While simple in concept, they were often wasteful, having an unnecessary amount of material in locations where stress may have been low. It is advantageous to have a sill in the form of a hollow section, of relatively thin walls, and to provide local reinforcement where required. It is also desirable that the hollow section be as manufactured at the mill, whether as tube or roll formed section, if possible, rather than welded. This often yields a saving in effort, may permit the use of a higher yield stress alloy, and may also reduce the number of defects or stress concentrations in the resulting structure. As the wall thickness decreases the prospect of buckling under buff loads and vertical loads increases, and measures to deter buckling would be advantageous. It would also be advantageous to provide protection for the sills to discourage damage to the sills due to clumsy loading of trailers or containers.
In the past, one method of dealing with areas of higher flange stresses in the side construction was to use a member of greater weight. As the thickness of structural members is reduced it would be advantageous to transfer loads from the railcar trucks to the bolsters, and thence to the side sills, more smoothly to discourage or reduce stress concentrations. One way to do this is to increase the depth of section at the bolster, with a consequent increase in height of the end decking.